In recent years, introduction of an FTTH (Fiber To The Home) plan has been started in many areas and, correspondingly, utilization of high-speed Internet in the home has been rapidly increasing. Under such a situation, enhancement of a fiber optics communication network has become necessary. To attain this, techniques such as a WDM (Wavelength Division Multiplex) scheme have come to be used.
In the WDM scheme, a transmission signal requires an optical power far higher than before. For example, orders of several Watts of optical power are required. Therefore, the edge face of a fiber optics for optical transmission may be burn out or a core may be burn out in an optical transmission line at a portion thereof with low resistance to an optical power, in some cases.
When a core is burn out in the optical transmission line at a portion with low resistance to an optical power, the burn-out portion spreads toward the light source direction in a chain reaction like a fuse wire, that is, fiber fuse phenomenon occurs (refer to, for example, Non-Patent Document 1 and Non-Patent Document 2).
Once such fire spreading due to the fiber fuse phenomenon occurs, it reaches up to a light source through a fusion spliced portion and a connector connection portion. This may lead eventually to breakage of devices such as a transmitter or amplifier.
It is thought that a factor causing the fiber fuse phenomenon includes high temperature, high optical power density, and light absorption by devices. At the fuse spliced portion, for example, the fiber fuse phenomenon is caused due to misalignment in the spliced portion. At the detachable connector point, a light is often absorbed by the stain of the edge face thereof, which increases the temperature to make it easy to cause the fiber fuse phenomenon. Since the fiber fuse phenomenon is thus easy to occur at the portion including a light absorber, a dopant contained in a core glass, such as Ge, can become a cause thereof.
Further, under the condition that the same light power is applied, a higher power density is obtained with a smaller effective sectional area of the propagation mode. The effective sectional area is nearly equal to a mode field diameter. Accordingly, a smaller mode field diameter allows easy occurrence of the fiber fuse phenomenon.
Several countermeasures have been taken to prevent the above fiber fuse phenomenon. For example, there is available a method that prevents occurrence of the fiber fuse phenomenon by expanding the core diameter of a single-mode fiber at the connector edge face (refer to, for example, Patent Document 1). Further, there is available an apparatus that, when the fiber fuse phenomenon occurs, uses a collimator lens to shut off the subsequent fire spreading (refer to, for example, Patent Document 2).
[Non Patent Document 1] The 2003 IEICE (Institute of Electronics, Information and Communication Engineers) General Conference, C-3-44, page 184
[Non Patent Document 2] Technical Digest of Optical Amplifiers & their applications
Topical meeting, Otaru, Japan, 2003 (Optical society of America, Washington, D.C.) TuC4, pages 193 to 195
[Patent Document 1] Jpn. Pat. Appln. Laid-Open Publication No. 2002-277685
[Patent Document 2] Jpn. Pat. Appln. Laid-Open Publication No. 2002-323639